1. Field of the Invention
This invention relates to a method for the production of a powder of fine inorganic particles excelling in dispersibility in various solvents. More particularly, it relates to a method for the production of a powder of fine inorganic particles containing very few cohering particles and attaining very easy dispersion in various solvents from a slurry of fine inorganic particles in a water-containing solvent by using a specific apparatus.
2. Description of the Prior Art
Heretofore, as means of producing a powder of fine inorganic particles, the method of mechanical pulverization which comprises subjecting lumps of powder to the action of a pulverization device in a solvent or in a non-solvent, the method of dry synthesis which produces fine particles by subjecting a gasifiable metal compound to reaction and decomposition in the gaseous phase or subjecting a metal plate as to spattering, and the method for producing a slurry of fine particles by the so-called wet synthesis as by hydrolyzing a hydrolyzable metal compound in a solution or subjecting a metal compound to the action of a precipitant or ion exchanger in a solution and thereafter separating the fine particles from the slurry and drying the separated fine particles have been known to the art.
The method of mechanical pulverization has a problem of requiring large energy for the pulverization, allowing only limited control of particle diameter, and suffering the produced particles to acquire no definite shape and a wide diameter distribution and exhibit susceptibility of agglomeration. The agglomeration of particles is particularly conspicuous when the production uses no solvent.
Though the method of dry synthesis is capable of producing very fine primary particles, the produced powder comprises secondary particles resulting from agglomeration of primary particles. Thus, this method has a problem of failing to produce fine discrete particles and suffering the produced particles to acquire a very high bulk specific gravity.
The method of wet synthesis has an advantage that the fine particles in the slurry are readily dispersible in a discrete form. Because this method requires complicated steps of separating particles from the slurry and drying the separated particles and further because the slurry contains water, this method has problems of suffering the produced particles to agglomerate readily during the component steps of process and necessitating great labor and a large expense for the production of a powder excellent in dispersibility.
As means of precluding the problem of agglomeration of produced particles mentioned above, the method which comprises adding an organic solvent to the slurry of fine particles, heating the slurry to a temperature exceeding the distillation temperature of water thereby dehydrating the slurry, separating the particles from the slurry or organic solvent, and drying the separated particles has been proposed. Though this method is effective in preventing the agglomeration of produced particles, it has a problem of requiring use of a large amount of organic solvent and moreover entailing extra steps of separating particles from the slurry and drying the separated particles. It also have possibility that the fine particles are still susceptible, if not always, to agglomeration during the component steps of process.
Japanese Patent Laid-Open SHO 62(1987)-138,310 discloses a method for producing a powder of particles free from agglomeration by preparing a slurry of fine metallic oxide particles from a metal alkoxide as a raw material and treating this slurry supercrytically with carbon dioxide gas. This method, however, has the disadvantage that the operation is difficult and the cost of apparatus is high.
As means of economically advantageously producing a powder of fine particles from a slurry of fine particles of varying pigment, a method has been proposed which makes use of a powderization apparatus similar to that which is used in the present invention (Japanese Patent Publication SHO 52(1977)-38,272). This method, however, has a disadvantage that when the slurry contains water, the produced powder contains numerous cohering particles.
For the solution of this problem, there has been proposed a method which causes an organic solvent which is insoluble in water and volatile to coexist in a slurry of fine particles of pigment when the slurry contains water (Japanese Patent Publication SHO 55(1980)-38,588). The our follow-up experiment conducted on this improved method has revealed that when the fine particles of pigment happen to be of an inorganic substance, the coexistence of the water-insoluble organic solvent in the slurry is very meagerly effective in preventing the agglomeration of particles. The effect aimed at is totally inconspicuous particularly when the fine particles happen to be those of metallic oxide possessing a very active surface as obtained by the method of wet synthesis, for example.
As means of treating the fine inorganic particles for imparting an organic quality to their surface, the method which produces a powder by adding such a surface-treating agent as an organic macromolecular compound, an alcohol, or a coupling agent, for example, to the slurry of fine inorganic particles obtained by a varying process, heating the slurry thereby effecting necessary surface treatment, and thereafter inducing expulsion of the solvent by evaporation has been known to the art.
Generally, the fine inorganic particles which have been given no surface treatment possess a highly hydrophilic surface and have a large amount of water adsorbed to the surface. Because of the surface quality mentioned above, the slurry necessitates use of water or a highly hydrophilic solvent as the dispersion medium. The slurry, therefore, is destined to allow the presence of a large amount of water in addition to the water adsorbed to the fine particles.
The adsorbed water and the coexisting water have been the cause for degrading the effect of the surface treatment performed on the fine particles by the surface-treating agent capable of reacting with and joining to the surfaces of the fine particles. It has been customary, therefore, to employ such methods of poor efficiency such as the method which relies on the use of a large amount of the surface-treating agent and the method which comprises preparatorily decreasing the adsorbed water and the coexisting water and subsequently giving the fine particles a surface treatment with the surface-treating agent. When the surface-treated fine particles are to be finished as a powder while in the presence of the adsorbed water and the coexisting water mentioned above, there arises a disadvantage that the fine particles are liable to agglomerate under the influence of these waters and the finished powder fails to acquire satisfactory dispersibility. As means of allowing the surface treatment to be performed on the fine particles without inducing the phenomenon of agglomeration, Japanese Patent Publication SHO 58(1983)-35,736, for example, discloses a method which comprises dispersing the fine particles in a solution of a surface-coating agent and treating the resultant dispersion in the same powderization apparatus as used in the present invention thereby subjecting the fine particles in the suspension to a surface-coating treatment. This method, however, has a problem of finding greatly limited utility and failing to manifest the effect of treatment fully because the treating agent is not chemically bound fast to the surfaces of the fine particles but is merely deposited physically on the surfaces and, therefore, is possibly peeled off the surfaces by the solvent in which the surface-treated fine particles are to be dispersed again for the convenience of use.
The basic difference between the method of the present invention and the known method described above resides in the fact that the surface treatment mentioned in the former method refers to what enables a compound possessing reactivity with the surfaces of the fines particles to be chemically bonded to the surfaces.
Moreover, the possibility that the compound capable of reacting with and joining to the surfaces of the fine inorganic particles will actually react with the surfaces under the conditions allowing the presence of a large amount of water in the slurry as a raw material and the conditions of production which realize this reaction are disclosed nowhere in any of the patent publications mentioned above.